It is known to those skilled in the art that an intramedullary nail has a general tendency to bend in the anterior-posterior direction during insertion into the bone, particularly in the femur. The anterior-posterior direction is perpendicular to the screw hole axis, and thus significantly affects the alignment between the drill-guiding hole in the drill guide and the nail screw hole. Medial-lateral bending on the other hand does not significantly affect drill-guiding alignment with the screw hole since the medial-lateral direction is parallel to the axis of the screw hole.
Other distal targeting systems using an external jig utilize some form of drill-guiding hole for each distal screw hole in the nail. The problem with these systems is that since there is no guarantee that the nail will not bend during insertion, there is always a possibility of misalignment between the distal screw hole and the drill-guiding hole, even assuming perfect alignment between the distal screw hole and the drill-guiding hole prior to nail insertion.
It has been observed that in cases where such distal jig systems fail to locate the distal screw hole, the drill (which is passed through the drill-guiding hole) is either anterior or posterior to the distal screw hole, at the same level (or distance from the proximal end of the nail) as the distal screw hole.
It has been further observed that in these cases, the drill is just touching the anterior or posterior surface of the nail when viewed under a fluoroscope. The explanation for this is that when the drill is misaligned with the screw hole, it hits the surface of the nail beside the screw hole. The inherent flexibility of the bone-nail-jig configuration then allows the nail and the drill to slip relative to each other so that the drill slides down to the anterior or posterior surface of the nail.